1. Field of the Invention
This invention relates to a stencil printer which comprises a printing drum, an internal press roller provided inside the printing drum to be brought into contact with the inner surface of the printing drum and a pinching means which is provided outside the printing drum and is pressed against the peripheral wall of the printing drum with a printing paper pinched therebetween, and more particularly to such a stencil printer in which the peripheral wall of the printing drum is prevented from being deformed inward through contact with the internal press roller.
2. Description of the Related Art
The structure of the conventional stencil printer and the drawbacks of the stencil printer will be described with reference to FIG. 12, hereinbelow. As shown in FIG. 12, the conventional stencil printer comprises a printing drum 200 having a cylindrical peripheral wall. The peripheral wall is permeable to ink and a stencil master is wound around the peripheral wall. The printing drum 200 is rotated about its longitudinal axis. Am arm 202 which is rotatable about a fixed shaft 201 is provided inside the printing drum 200. An internal press roller 203 is mounted for rotation on the arm 202. A doctor roller 205 is disposed near the internal press roller 203. The doctor roller 205 forms an ink layer of a predetermined thickness on the outer surface of the internal press roller 203. The arm 202 is rotated in synchronization with rotation of the printing drum 200 so that the internal press roller 203 presses outward the peripheral wall of the printing drum 200. A paper pinch drum 206 is supported for rotation at a predetermined distance from the printing drum 200. When a printing paper is supplied between the printing drum 200 and the paper pinch drum 206, the internal press roller 203 deforms outward the peripheral wall of the printing drum 200 and the printing paper is conveyed pinched between the stencil master on the deformed part of the peripheral wall and the paper pinch drum 206, whereby ink supplied from the internal press roller 203 to the inner surface of the peripheral wall of the printing drum 200 passes through the peripheral wall of the printing drum 200 and the stencil master and is transferred to the printing paper to form an image.
In the conventional stencil printer shown in FIG. 12, there has been a problem that a part of the peripheral wall of the printing drum 200 adheres to the internal press roller 203 and is deformed inward as indicated at D in FIG. 12. This phenomenon occurs on the downstream side of the contact line between the outer surface of the internal press roller 203 and inner surface of the peripheral wall of the printing drum 200 and occurs due to the fact that a part of the inner surface of the peripheral wall of the printing drum 200 cannot be properly separated from the outer surface of the internal press roller 203 after the part passes through the contact line between the internal press roller 203 and the paper pinch drum 206.
Accordingly, this phenomenon is more apt to occur when the peripheral wall of the printing drum 200 is deformable or when the ink has a high viscosity. This phenomenon is especially apt to occur under a low temperature where the viscosity of the ink increases. This phenomenon can occur not only in the conventional stencil printer where the peripheral wall of the printing drum 200 is flexible but also in stencil printers where the peripheral wall of the printing drum is rigid. This is because the thickness of the rigid peripheral wall of the printing drum is made as thin as possible in order to reduce the amount of ink held by the printing drum so that printing quality is improved. For example, when the peripheral wall of the printing drum is formed of a stainless steel plate which is about 0.15 mm in thickness, the aforesaid deformation of the peripheral wall can occur depending on the conditions.
The aforesaid deformation of the peripheral wall of the printing drum can cause the following problems. That is, when the peripheral wall of the printing drum is deformed, the stencil master attached on the outer surface of the peripheral wall can be stretched or displaced in the circumferential direction of the printing drum. Further when the peripheral wall is repeatedly deformed and is repeatedly subjected to stress, the peripheral wall can be broken at the part where the stress is concentrated. Further repeated deformation of the peripheral wall can finally result in plastic deformation of the peripheral wall, e.g., the peripheral wall can corrugate as seen in a cross-section parallel to the longitudinal axis of the printing drum. Further when the peripheral wall is finally moved away from the internal press roller under its resiliency, ink splashes mechanisms in the printing drum.